Phase Inverter – What it Does and How it Works

Of all the circuits in a tube amplifier, the Phase Inverter, also known is the Phase Splitter, is the most difficult to understand by even some experienced techs. Its function is relatively simple: take a signal input, and create two outputs, one that is identical (e.g. in-phase) to the original, and another that is a mirror-image (phase-inverted or flipped phase). Each signal feeds a power tube (or bank of power tubes) that is connected to each side of the Output Transformer’s primary winding in the typical Push-Pull configuration. Single-ended power amp, like those contained in the Fender Champ, which sport only one power tube, do not require this additional step, and need only a driver before the power tube to boost the preamp signal to a level usable by the lone power tube.

So, why is the Push-Pull method of power amplification used when it is inherently more complex and costly? Several reasons. First of all, it enables us to use a more efficient amplifier class called Class AB. Whereas single-ended audio amplifiers ALWAYS run in the Class A mode, which runs the tube constantly at maximum power (thereby shortening it’s lifespan), Class AB runs each tube only slightly above the lowest operational point (called “idle”), and each one is called on as needed to deliver power when necessary. No signal in, no power use, therefore the tubes remain relatively cool until pushed.

The second reason for using Push-Pull is that unwanted sonic artifacts, such as hum and odd-harmonic distortion (this is the nasty, raspy kind), are naturally cancelled in the Output Transformer. Even-order harmonic distortion (the kind that sounds cool), remains relatively untouched.

Back to the Phase Inverter. There are two designs that dominate guitar amps. One is the “Cathodyne”, also known as the “Split-Load” (Fig. 1), and the other is the “Long-Tailed Pair”, derived from a circuit called the Schmitt Inverter (Fig. 2).

The Split-Load is the simplest arrangement. It splits the signal by virtue of the fact that the signal appearing at the cathode of the tube is in-phase (this circuit by itself is called a “Cathode Follower”), while the signal at the plate is out-of-phase (this is a typical Common-Cathode amplifier). The way this works is that signal input to the tube at the grid causes a variation in current flow from the cathode to the plate, producing a voltage swing on the plate, out-of-phase with the input signal. It is also important to know that the variation in current ALSO appears at the cathode, as a signal which is IN-PHASE with the input signal. As long as the plate and cathode resistors are the same value, the amplitude of the two outputs will be similar, except for the flipped-phase. This is a very important concept to grasp for later. The main drawback of this circuit is that it offers no signal voltage gain. What you put in is what you get out, except one side is flipped-phase. Therefore, an additional tube stage called a “Driver” is used ahead of it. The Driver provides the gain, the Cathodyne inverter provides the necessary phase-flip, and they both live as a happy family.

But wait….what if you could do all that with ONE circuit? Well, you can. Enter the “Long-Tailed Pair”, presumably called as such due to the dual resistor “tail” used for critical bias of the dual-triode tube.

I’d like to preface this by listing the three ways we can use a triode tube: Common-Cathode (where the cathode is grounded and signal is fed to the grid, the most common type of triode amplifier), Common-Grid (where the grid is grounded and signal is fed to the cathode), and Common-Anode (where the plate is “grounded”, not to 0V, but directly to the power supply, which is a “virtual ground”, also called a “Cathode Follower” and, rarely, a “Buffer”). We already know that the Cathode Follower does NOT provide signal voltage gain. It DOES deliver current gain, which is good for circuits like tone stacks that tend to hog current. However, I digress: the OTHER two amplifer arrangements, Common-Cathode and Common-Grid DO provide voltage gain, though the Common-Grid amplifier isn’t quite as effective as the Common-Cathode amplifier in doing this. The gain for a given signal input is LESS than that of the Common-Cathode circuit. Save this thought for later.

What the “Long-Tailed Pair” does is use two triodes, one in a Common-Cathode arrangement, the other as a Common-Grid amp. The signal enters the first stage (Common-Cathode) in the usual way, through the grid. This produces a voltage swing on the plate AND the cathode of this stage, as described earlier. The second-stage, which has a common (grounded) grid, has it’s output at the plate, like the first stage, so…..just HOW do we feed signal into it? Well, the cathode is still up for grabs. This is where it gets mighty ingenious. If you tie the cathode of the first stage to the cathode of the second stage, the variation in current of the first stage will be superimposed on the cathodes of the second stage. Here is the circuit again, redrawn and simplified in Fig 3:

Basically, what is happening here is that the plate circuit, which has voltage gain, sends it’s signal to the power tubes. The cathode circuit, which is essentially a cathode follower, we already know has NO voltage gain, so the second stage Common-Grid amp provides the gain. This is the key element missing in the Cathodyne Phase Inverter (Fig. 1). After voltage gain is applied, the signal then travels to the power tubes as well.

I stated earlier that the Common-Cathode amp has higher voltage gain than that of a Common-Grid amp. In an attempt to balance this out, the plate resistor of the Common-Cathode circuit is slightly reduced, reducing the gain of that stage. In many tube amps, it will be reduced to 82K vs. 100K for the second stage. All else remaining equal, lowering the plate resistor value also lowers the gain of the stage. The secondary effect of this is that the actual signal is not balanced on both sides, making the output somewhat asymmetrical (i.e. the positive signal swing is not equal to the negative signal swing). There are a few amp companies that ignore this and use 100K resistors for both, introducing a DIFFERENT kind of asymmetry into the signal because of a DC shift. And the Cathodyne circuit? It isn’t perfectly symmetrical either, because the source impedances (i.e. the cathode and anode/plate) are different. One (the plate) is high, delivering lots of voltage, but not much current, while the other (cathode) is low, delivering lots of current, but no voltage gain. The bottom line is:

THERE IS NO SUCH THING AS A PERFECTLY BALANCED PHASE INVERTER!!!

You could match the sections of a dual-triode, match the caps and resistors in the circuit, and all will still be imperfect. But, my friends, guitar amps are NOT hi-fi amps, nor do we want them to be. They are not about audio perfection, they are about tone, and the reality is that the inherent imbalance produced in the Phase Inverter results in more harmonic complexity, resulting in a more sonically pleasing end result. Remove the imperfections, and you sterilize the amp. You know, sometimes even people seem a lot more real with a few wrinkles on their face!

Now, go in peace. Tonight, you amp junkies can sleep, knowing that, together, we have solved one of the great mysteries of life!!!

John R. Frondelli is the Technical Services Director at DBM Pro Audio In New York. He has been a technician for 30 years and has repaired, restored and custom built all types of musical equipment. Part of his client list includes Bob Dylan, Lenny Kravitz, U2 and The Who. �